Has anyone successfully repaired bimetal contacts in a dash gauge

[240260280z]

I made 3 attempts but the contacts carboned up:

1. Caig Deoxit

2. Burnishing

3. Burnishing and Deoxit

I am wondering if twisting and shifting will cause two new fresh (and presumably plated) parts of the contacts to touch?

[EScanlon]

I think this is where we need folks like SteveJ and PSDenno (and I may have those names completely wrong) but I'm referring to those guys who are actually working in Engineering fields and preferably Electrical Engineering (or at least a good knowledge of electrickery).

With the knowledge I've gained from the Roadster, I'm wondering if that contact isn't an attempt at replacing the Voltage Regulator (not the one in the engine bay) that the earlier vehicles required for the Fuel and Temp gauges.

Thoughts?

E

[240260280z]

Yup. I've seen references as "vibrating" contact. I have watched it and I do see a spark when it contacts. Not good for carbon buildup.

The thing will work for ~ 30min then it slowly goes open.

[Mikes Z car]

I wonder if it would be practical to put a heavy duty transistor in place that would use the contacts as input to the transistor to take the electric load off of the contacts with the output of the transistor in series with the bimetallic metal that drives the needle? Pure speculation on my part as I don't really understand how the bimetallic strip makes the needle turn!

Hmmm I wonder if this could be used to extend the life of the clock with the same idea of taking the load off of the contacts.

[240260280z]

Just found this warning that confirms my worry about plating:

Do NOT burnish or file low-current relay contacts!

Using any abrasive, even a mild abrasive on low or medium current relay contacts will damage the relay.

Low or medium current relays almost always have a thin gold or silver flash. This overlay prevents oxidation and improves low current connections. The flash or plating is easily removed, and if it is removed, low signal level connection issues will reappear quicker.

Edited October 2, 2012 by Blue

[240260280z]

Before Bending

After Bend)ing (new contact area

[EScanlon]

What I recall from the discussions on the Roadster's gauges is that they require a pulsedd voltage at 8v. That explanation there will probably make more sense to Steve and PSDenno than me, but if I understood it properly, it was reduced voltage (8v) at intervals that allows the gauges (fuel and temp) to check the status of their sensors and therefore "read" the position required. That is, if it had 12v continuously, it would either max out the gauge (like when we short the lead to see if the gauge is working) or simply not register at all.

Having had a VR go wanky in my Roadster, I can tell you that both gauges will give you all sorts of weird and inaccurate readings... meaning, totally useless.

With that thought in mind, is where I make the possible connection to the contacts in the Z's gauges.

I'll send both those guys a PM and ask them to look at this and offer their 2¢

E

[SteveJ]

Okay, you guys goaded me into doing some research.

I found this link interesting: http://autoclub.atilim.edu.tr/dokuman/Automotive%20Electrical%20and%20Electronic%20Systems/Chapter13.pdf

I took apart a faulty fuel gauge I had.

I thought there was a voltage divider at play, and I figured it out...I think. (Correction as I type...It's a current limiter. The bimetallic strips are in parallel, so they both have 12 volts when connected.)

The gauge has two bimetallic strips that can be heated up. One moves the needle and the other limits the current.

The one that limits the current does so in the manner Enrique suggests. It heats up and breaks contact, allowing the full current to go to the sender side. This lets the other bimetallic strip heat up as it moves the needle.

The bimetallic strip on the current limiter cools down, and the contacts make, letting it heat up again. This takes current away from the gauge bimetallic strip.

If the contacts carbon up on the current limiter, that could cause too much current to flow through the gauge bimetallic strip for too long.

Maybe I'll set up my test bench and multimeters on Saturday and let it be an electrical engineering play day.

Thanks for getting me to think, guys. It's about time I spent some quality time on car stuff.

Edited October 3, 2012 by SteveJ
clearing up some poor grammar

[Captain Obvious]

I've never looked inside the gauges, so I'm shooting from the hip...

Put a small capacitor across the arcing contacts to act as a snubber to prevent future arcing and use some gold leaf from the craft store burnished onto the contacts to provide a better contact?

[Captain Obvious]

And another suggestion...

I've got a spare 77 gauge that works fine, and you're welcome to have it if you want. Don't know if it's the same as what you're working on, but if it would help, just let me know!

[240260280z]

Hey CO. Sure! I hope to be in NJ soon so I'll take some time to come over.

[Captain Obvious]

Sweet. Looking forward to it.

So, the gauge I have works fine, and I know that because I used it for a while in my car until I swapped out my entire dash for one with "fewer" cracks. Now all I have to do is keep my mitts off it until you get here. Problem is you've got me all curious now!!

So to quell my curiosity, I did a few minutes of armchair research and I believe the bi-metallic strip with the contacts (not the one that moves the needle, but the other one) is a temperature compensation device in addition to a voltage regulator. It pulse width modulates the current through the other strip in response to changes in ambient temperature and system voltage.

http://home.roadrunner.com/~john.strenk/Gauge%20web%20page/WebPageGauges1.htm

Remember though, I still haven't been inside one myself. And I promise to TRY to keep it that way until you get here!